In a recent work [1] we studied the electron transport in gated surface states of a 3D Topological Insulators. In the present work we have now theoretically investigated the modulation of transport properties of such gated surface states of a 3D topological Insulator under a terahertz field. We calculate the transmittance of surface electrons using the Floquet scattering matrix formalism. The resulting transmittance contains the contribution both from elastic and inelastic scattering processes and show clear signature of Klein tunneling . As expected the contribution of the Floquet sidebands to the total transmission increases as we increase/decrease the amplitude/frequency of modulating field. It is shown that the Fabry-Perot resonances in the transmission and correspondingly in the conductivity show significant modification under the action of such time dependent field that can be used in potential optoelectronic devices.